In a manufacturing process of a semiconductor device or an FPD (flat panel display), a plasma is often used in the process, e.g., etching, deposition, oxidation, sputtering or the like, in order to allow a processing gas to react efficiently at a relatively low temperature. Conventionally, a capacitively coupled plasma processing apparatus is mainly used to easily realize a plasma having a large diameter for a single-wafer plasma processing apparatus.
In general, in a capacitively coupled plasma processing apparatus, an upper electrode and a lower electrode are disposed in parallel with each other in a vacuum processing chamber, and a target substrate (e.g., a semiconductor wafer, a glass substrate or the like) is mounted on the lower electrode, while a radio frequency (RF) power is applied between both electrodes. Then, electrons accelerated by an RF electric field formed between the electrodes, electrons emitted from the electrodes, or heated electrons collide with molecules of a processing gas to ionize them to thereby generate plasma of the processing gas, and accordingly, a desired microprocessing, e.g., etching, is performed on a substrate surface by radicals and ions in the plasma.
Here, the electrode to which the RF power is applied serves as a cathode (negative pole) that is connected to an RF power supply via a blocking capacitor in a matching unit. A cathode coupling type in which an RF power is applied to the lower electrode which mounts thereon a substrate and serves as a cathode can perform a well directed anisotropic etching by substantially vertically attracting ions in the plasma toward the substrate by using a self-bias voltage generated in the lower electrode.
Along with the recent trend for miniaturization of a design rule in manufacturing a semiconductor device or the like, an ever increasingly high dimensional accuracy is required especially in the plasma etching and, hence, selectivity against an etching mask and an underlying layer and/or in-plane uniformity in the etching has to be improved. Accordingly, there arises a demand for lowering ion energy as well as pressure in a processing region inside the chamber. For that reason, an RF power of about 40 MHz or greater has been applied, which is significantly higher than that applied in a conventional case.
Here, it becomes difficult to make a plasma of a uniform density in a processing space of the chamber (particularly in a radial direction). In other words, when the frequency of the RF power for plasma generation is increased, the plasma density becomes non-uniform by having a mountain-shaped profile in which the plasma density is maximized mostly above a central portion of a substrate and is minimized mostly above an edge portion of the substrate by a wavelength effect by which a standing wave is produced in the chamber and/or a skin effect by which the RF power is concentrated on a central portion of an electrode surface. If the plasma density is non-uniform above the substrate, a plasma process also becomes non-uniform, which leads to a reduced production yield of devices.
Various studies on electrode structures have been made to overcome such a problem. For example, Japanese Patent Laid-open Application No. 2004-363552 (Corresponding to U.S. Patent Application Publication No. 2005/0276928 A1) discloses a plasma processing apparatus in which a dielectric material is embedded at a main surface of an electrode facing a processing space and impedance of the RF power emitted from the electrode main surface to the processing space is made to be relatively large at a central portion of the electrode and relatively small at an edge portion of the electrode, thereby improving uniformity of a plasma density distribution.
At a certain frequency range, the method of embedding the dielectric material at the electrode main surface as described above can be employed to effectively transform, to a flat (uniform) profile, a mountain-like profile of the plasma density distribution on a subject substrate, which has its peak at the central portion of the substrate and becomes gradually getting low toward an edge portion of the substrate. However, if a frequency of the employed RF power is increased further, variation of the plasma density distribution (altitude difference in the mountain-like distribution) becomes larger in proportion to the increased frequency, thereby making it difficult to flattening the plasma density distribution. In addition, a cathode-coupled plasma processing apparatus is disadvantageous in that, if a frequency of the RF power exceeds about 80 MHz, a plasma density distribution produced by an RF power of a certain power level becomes to have a W-like profile in which the plasma density is high above the central portion and the edge portion of a substrate and low above the portion therebetween. Such a W-like profile cannot be dealt with the method of flattening the mountain-like profile.